1. Field of the Invention
The present invention relates to a method of forming a piezoelectric layer which includes a ferroelectric film and which constitutes a piezoelectric element, and relates to a piezoelectric layer and a piezoelectric element. Further, the present invention relates to a heating apparatus used in degreasing a ferroelectric precursor film, which becomes a piezoelectric layer.
2. Description of the Related Art
Piezoelectric elements having piezoelectric layers made of ferroelectric films have spontaneous polarization, high dielectric constants, electrooptical effect, piezoelectric effect, pyroelectric effect, and the like. Accordingly, piezoelectric elements are applied to a wide range of devices, e.g., an ink jet recording head and the like. The ferroelectric film (piezoelectric layer) constituting such a piezoelectric element is formed by, for example, the sol-gel method. That is, a ferroelectric precursor film is formed by applying, for example, a sol of an organometallic compound to the top of a predetermined substrate, and the ferroelectric precursor film is heated to a predetermined temperature to be dried and degreased and is then baked at a higher temperature to be crystallized, thus obtaining the ferroelectric film.
Here, when the ferroelectric precursor film is degreased, the ferroelectric precursor film needs to be heated to a predetermined temperature. As a heating apparatus used in this case, for example, there is one in which a substrate is supported by proximity pins placed at a position facing a hot plate to be vertically movable and in which a heating temperature of the substrate is adjusted by lowering the proximity pins and thus adjusting the distance between the substrate and the hot plate (e.g., refer to Japanese Unexamined Patent Publication No. Hei 6(1994)-181173).
However, when the ferroelectric precursor film is degreased by use of such a heating apparatus, the ferroelectric precursor film may not be sufficiently degreased. That is, since the heating apparatus is placed, for example, inside a housing or the like for safety and the like, a temperature of the entire inside of the housing increases when the hot plate is heated. This causes a problem in which a coating is formed on the surface of the ferroelectric precursor film and in which the entire ferroelectric precursor film cannot be sufficiently degreased.
Moreover, if degreasing is performed by use of such a heating apparatus, there is a problem in which it is difficult for crystal nuclei to be formed in the ferroelectric precursor film and in which the ferroelectric precursor film is not favorably crystallized even when the ferroelectric precursor film is baked. This may be because a temperature of the ferroelectric precursor film sharply increases.